1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method suited to an observation of the dynamic state of a moving material in a body such as blood or cerebrospinal fluid (CSF).
2. Description of the Related Art
A magnetic resonance imaging apparatus is an apparatus that excites nuclear spins of atoms in a subject magnetically with a radio-frequency signal having the Larmor frequency and acquires an MR signal generated due to the excitation, thereby reconstructing an MRI image or an MRA image (hereinafter, collectively referred to as an MR image).
In recent years, in the art of the magnetic resonance imaging apparatus, a method is known in which inversion recovery (IR) pulses are applied to thereby perform labeling (tagging) on an observation target in the form of longitudinal magnetization in both time and space domains, and MR images are taken after a given time (for example, see “Considerations of Magnetic Resonance Angiography by Selective Inversion Recovery,” D. G. Nishimura et al., Magnetic Resonance in Medicine, Vol. 7, pp. 472-484, 1988). By observing the images obtained by this method, it is possible to visually recognize the distribution of labeled observation target.
Another method is known which conducts labeling with longitudinal magnetization by a selective excitation method before echo signal collection for imaging, and takes a plurality of images while varying a time lapse TI from the labeling to the start of the imaging (for example, see Japanese Laid-open (Kokai) Patent Publication No. 2001-252263). The plural images obtained by this method are sequentially displayed at regular intervals, and therefore, it is possible to observe the dynamic state of a moving material in a body such as blood or cerebrospinal fluid.
Depending on portions of the body, however, blood may flow into the portion from multiple directions. When such a portion is included in an imaging region, it is difficult from the conventional method to label only the blood vessel that is to be diagnosed. Moreover, the velocity of blood flowing into the image region differs from blood vessel to blood vessel through which the blood flows. For this reason, the time lapse from the labeling to the start of the imaging is not appropriately set, with the result that a blood vessel that is not intended to be diagnosed is also imaged, making it difficult to observe the blood vessel that is to be diagnosed.